[AIE] Handle large stack frames in adjustSPReg

[atassis]

Problem

adjustSPReg emits the SP adjustment via the sp_imm pointer-add form, which encodes a limited immediate (+-2^18 bytes on AIE2P/AIE2PS, +-2^17 on AIE2). For a larger frame it took a report_fatal_error(...) path, crashing the backend instead of generating code. This is reachable whenever a function needs a frame that large, e.g. at -O0 where large vector temporaries spill to the stack. Minimal repro (aie2p):

define void @big_frame(ptr %out) {
  %buf = alloca [300000 x i8], align 64
  %p = getelementptr [300000 x i8], ptr %buf, i64 0, i64 0
  store volatile i8 42, ptr %p
  ret void
}

llc -O0 -mtriple=aie2p on that hits the fatal error.

Fix

Materialize the offset in an eM modifier register with MOVXM, then apply it with a register-form pointer add:

• AIE2P / AIE2PS: PADDXM_pstm_sp (paddxm [sp], $mod), the register-operand sibling of the PADDXM_pstm_sp_imm used for small frames. It defs/uses SP implicitly, so the reserved SP register is not an explicit operand.
• AIE2: no register-form SP add exists and SP is reserved, so copy SP into a scratch pointer register (MOV_mv_scl), add the offset there (PADDA_lda_ptr_inc_idx), and copy back to SP. This is the same sequence the backend already emits to update SP for a dynamic stack allocation.

The virtual scratch registers are resolved by the register scavenger (requiresRegisterScavenging is true on all three targets); adjustReg and the dynamic-alloca lowering already rely on the identical patterns.

Testing

New tests llvm/test/CodeGen/AIE/{aie2p,aie2ps,aie2}/large-stack-frame.ll (run with -verify-machineinstrs). Each checks that a larger-than-range frame emits the register-form SP adjust and that an in-range frame still uses the compact sp_imm form (no regression). Verified for all three: the minimal repro that crashed now compiles, passes -verify-machineinstrs, and assembles to a valid object; the existing prolog/epilog and dyn-stackalloc tests still pass.

Notes

• AIE1 is out of scope: its own adjustReg also report_fatal_errors on large offsets (no register-materialization pattern to mirror), and it is the legacy target.
• MOVXM encodes a 32-bit signed immediate, which covers any representable stack frame.